This Application is a U.S. National Phase Application of PCT International Application PCT/JP00/01031.
1. Field of the Invention
The present invention relates to a piezoelectric vibration element relating to thickness mode such as piezoelectric vibration element and piezoelectric filter in which the overtone mode is the main vibration.
2. Background of the Invention
As a piezoelectric vibration element (hereinafter simply referred to as PVE) using bulk waves which are waves propagating in a solid matter, the PVE used as clock source for various electronic appliances and the piezoelectric filter used for frequency extraction of communication appliances are known.
In the recent trend of higher density, higher speed and larger capacity in electronic appliances, the frequency region used in such PVE is shifted to the higher frequency side. As the vibration mode, the thickness vibration such as thickness slide direction and thickness longitudinal direction is often employed. However, since the resonance frequency of the PVE using the thickness vibration is inversely proportional to the plate thickness of the element, thinning of element plate is indispensable for heightening of frequency. In the element thinning process, the element machining method must be determined in consideration of the mechanical strength of the piezoelectric materials, machining precision such as parallelism of plate thickness and distribution within the plane, and reduction of cost. In an ordinary method, the limit of plate thickness of piezoelectric material is about 50 xcexcm. Converting this value of plate thickness into frequency, in the case of vibration of lowest order wave mode, even when the thickness longitudinal vibration of fast vibration transmission speed is used, the upper limit of the vibration frequency is about 70 MHz. In the case of a higher frequency, the overtone mode is obtained, that is, the wavelength in the thickness direction is (2n+1) times (n being a natural number) of the lowest order wave mode (hereinafter simply referred to as LOWM). Herein, the LOWM is often the first-order mode usually having a half wavelength in the thickness direction.
As the prior art, the polarized inverted layer forming technique of lithium niobate is disclosed in Japanese Laid-open Patent No. 63-311808, and the direct bonding technique of piezoelectric single crystal substrate is disclosed in xe2x80x9cVariable Property Crystal Resonators by Direct Bonding Techniquesxe2x80x9d in IEEE Ultrasonics Symposium, pp. 897-900, 1996.
In a structure in which the axis of polarization is inverted in the center of the thickness direction, the LOWM may be a second-order mode having one wavelength in the thickness direction. That is, when the LOWM is first-order, the overtone mode is third-order, fifth-order, seventh-order, and so forth. When the LOWM is second-order, the overtone mode is sixth-order, tenth-order, fourteenth-order, and so forth. However, even when the overtone mode is used, there is an LOWM at the low frequency side. Accordingly, if suppression of LOWM is imperfect, the oscillation frequency may jump or other abnormal oscillation may occur in an oscillation circuit using a PVE.
Herein, methods for suppressing the LOWM include:
A method of suppressing the LOWM by circuit configuration of oscillation circuit or the like, so that a desired order number may be utilized; and
A method of suppressing the LOWM by material or structure of the PVE.
The method of suppressing the LOWM by circuit configuration includes the overtone mode oscillation circuit. In this circuit, an LC resonance circuit of coil and capacitor is added to the oscillation circuit used in the LOWM. By adding this LC resonance circuit, it is designed to tune in the mode of a desired order, and oscillate in the overtone mode. As other circuit configuration, the same operation is possible in a Colpitts oscillator using transistors.
On the other hand, the method of suppressing the LOWM by the PVE itself includes a method of selecting such a piezoelectric material that the LOWM is hardly excited as compared with the overtone mode, and a method of suppressing the LOWM by the structure of the vibration element.
In the PVE or piezoelectric filter, it is sometimes required to use a specific vibration mode such as thickness slide vibration and thickness longitudinal vibration in a specific piezoelectric material. In such a case, when an excitation electrode is formed partially on the PVE principal surface, it is known that the vibration energy is confined only underneath the excitation electrode. Such phenomenon is known as energy confinement, which has been specifically analyzed by Shockley, Ogami and others. The energy confinement is described below by referring to the sectional view of the PVE in FIG. 8. As shown in FIG. 8, supposing the cut-off frequency in a certain portion of an excitation electrode 8 to be F 0, and the cut-off frequency in the non-electrode portion to be F 0xe2x80x2. In this case, at the frequency larger than F 0xe2x80x2, the vibration energy freely propagates a vibration element 7. Even underneath the excitation electrode, its frequency does not form a stationary wave. However, at a frequency larger than F 0 and smaller than F 0xe2x80x2, the vibration energy propagates freely in a certain portion of the excitation electrode 8. In the non-electrode portion, however, the vibration energy attenuates exponentially. Therefore, the vibration displacement is smaller as going toward the PVE end portion. As a result, the vibration energy is concentrated near the excitation electrode. Depending on the piezoelectric material, however, not satisfying the condition of energy confinement of LOWM, the condition of energy confinement is satisfied in the overtone mode. In this case, too, the mode to be excited most is the third-order overtone mode having a wavelength of three times in the thickness direction if the LOWM is first-order. Such materials include piezoelectric ceramics such as lead titanate material, lithium niobate 36xc2x0 rotation Y-plate, and lithium niobate Z-plate. Using these materials, the PVE using the thickness longitudinal third-order overtone mode is developed practically. However; even if using a piezoelectric material of which LOWM does not satisfy the condition for energy confinement, a method of utilizing the overtone mode stably is disclosed. (for example, xe2x80x9cThickness longitudinal mode third-order harmonic vibrator using LiNbO3 36xc2x0 rotation Y-plate, published in Journal of Society of Electronics, Information and Communication, Vol. J81-C-1, No. 6, pp. 357-364, 1998). The method disclosed in this literature requires optimization of dimensions of excitation electrode and holding method of PVE so as to suppress the LOWM excitation sufficiently.
Or, when using a piezoelectric material of which LOWM satisfies the energy confinement condition, such as crystal AT plate, there is a method of using the overtone mode stably regardless of the circuit configuration. In this method, it is required to suppress the LOWM by the structure of the PVE. For example, by making use of the difference in the degree of energy confinement between the LOWM and overtone mode, an element forming an electrode of a certain mass near the PVE is known (for example, xe2x80x9cThird-order overtone mode crystal vibrator suppressing fundamental wave mode (EFD vibrator)xe2x80x9d disclosed in Toyo Communication Equipment Technical Bulletin No. 42, pp. 1-7, 1998). This principle is explained by referring to FIG. 9. FIG. 9 shows results of calculation of the relative displacement of LOWM and overtone mode in the PVE longitudinal direction. In the diagram, the axis of abscissas denotes the distance from the central part of the vibration element to the outer circumference, and the axis of ordinates represents the relative displacement supposing the vibration displacement in the center of the PVE to be 1. Hereinafter, this diagram is called the relative displacement profile. As mentioned in FIG. 8, by making use of the energy confinement, the vibration shows a sinusoidal wave underneath the excitation electrode. In the non-electrode portion, the vibration displacement attenuates exponentially. In FIG. 9, supposing the displacement of the element center to be 1, the relative displacement profile of the PVE having a set of electrodes becomes smaller as going away from the center. As shown in FIG. 9, the third-order overtone mode is superior in energy confinement than the LOWM. Therefore, the vibration displacement of the third-order overtone mode is distributed mostly underneath the excitation electrode. As known herein, however, the LOWM is large in the vibration displacement in the non-electrode portion, and the attenuation is insufficient. In the piezoelectric vibrator shown in FIGS. 10A, 10B, the vibration displacement of LOWM is large, and the mass of the electrode 9 and others is added to the region in which the vibration displacement of the overtone mode is almost 0, that is, in the periphery of the PVE. In the piezoelectric vibrator of this constitution, by lowering the cut-off frequency of the LOWM in the portion of the electrode 9, the vibration displacement of the LOWM is propagated to the outer circumference to leak the vibration energy, thereby suppressing the excitation level of the LOWM.
When using the overtone mode by the circuit configuration, as compared with the case of using the LOWM, the following problems were involves, that is,
The number of parts is increased, such as coils and capacitors;
It is difficult to form in an IC because coils are used; and
It is difficult to realize an adjustment-free constitution.
Or in the case of use of piezoelectric material of which LOWM satisfies the energy confinement condition in the PVE, as a matter of course, the LOWM excitation level is higher than that of the overtone mode. Accordingly, as a method of suppressing the LOWM, it is known to make use of the difference in degree of energy confinement between the LOWM and overtone mode. This is a method of adding an electrode of a certain mass in the PVE peripheral area in the region where the vibration displacement of the LOWM is large and the vibration displacement of the overtone mode is almost 0. In this case, the cut-off frequency of the PVE peripheral area is changed due to oxidation of electrode material or change of interface of electrode film. By this change of cut-off frequency, the PVE by this method is unstable in propagation amount of vibration energy, and lacks in long-term reliability.
Or if the piezoelectric material of which LOWM does not satisfy the energy confinement condition is used in the PVE, in case of an insufficient suppression of LOWM, the PVE may induce oscillation frequency jump or other abnormal oscillation.
The invention is intended to solve the problems of the prior art, and it is hence an object thereof to present a PVE capable of suppressing the LOWM efficiently and allowing to utilize the overtone mode stably if made of a piezoelectric material of which LOWM satisfies the energy confinement condition, and also excellent in long-term reliability because the vibration element part and vibration propagation part are made of same material.
To achieve the object, the PVE of the invention comprises:
a) a vibration element part,
b) a vibration propagation part made of a same material with a greater thickness than the vibration element part,
c) a groove disposed in the boundary of the vibration element part and vibration propagation part,
d) excitation electrodes confronting the face side and back side of the vibration element part,
e) a connection electrode disposed in the vibration propagation part, and
f) an external lead-out electrode connected electrically to the connection electrode.
The PVE having such constitution mainly vibrates in the overtone mode of which wavelength in the thickness direction is (2n+1) times (n being a natural number) of LOWM when the LOWM is first-order or second-order.
In this constitution, the PVE of the invention brings about the following two effects:
An effect of leak of vibration energy of LOWM through the vibration propagation part; and
An effect of increasing the resonance impedance by disturbing the resonance of LOWM as the reflected wave of vibration occurring in the groove portion induces an undesired vibration.
Owing to these two effects, the PVE of the invention is:
1. Capable of suppressing the LOWM efficiently if using a piezoelectric material of which LOWM satisfies the energy confinement condition;
2. Excellent in long-term reliability because the vibration element part and vibration propagation part are made of a same material; and
3. Decreased in the number of parts because a tuning circuit of coil and capacitor is not needed in the oscillation circuit.
The ratio 2h/H, where h is the depth of the groove in the boundary of the vibration element part and vibration propagation part of the PVE of the invention and H is the thickness of the vibration element part, is defined to be 2/3 or less.
The main vibration of the PVE of the invention is the thickness slide vibration, and the vibration element part and vibration propagation part are integrated only in the vibration propagation direction.
The vibration propagation part of the PVE of the invention is formed by applying a vibration-absorbing material on its outer circumference.
The vibration-absorbing material of the PVE of the invention is a conductive material.
The vibration propagation part of the PVE of the invention has an upper lid or a lower lid, or both of them.
The vibration element part of the PVE of the invention has at least a pair of input electrode and output electrode at one side of its principal plane, and a grounding electrode at other principal plane.